U.S. Pat. No. 3,927,967 discloses the use of photoactivator dyes like phtalocyanine dyes for use in detergents. Such photoactivator dyes are capable of absorbing electromagnetic radiation in the visible light range and releasing the absorbed energy quanta in a form that provides bleaching action on fabrics. It is believed that the energy quanta form singlet oxygen which is oxidatively active. Some photoactivators, like zinc phtalocyanine sulfonate (ZPS) and aluminium phtalocyanine sulfonate (APS), have found commercial application in granular detergent compositions.
However, when used in detergent compositions the phtalocyanine dyes present certain drawbacks that have heretofore not properly been addressed. One such drawback is that the particles in which the dyes are incorporated do not dissolve sufficiently quickly when contacted with water to avoid dye staining. Such contact is particularly likely to occur in endwise situations and even when the detergent composition containing the dye is dispensed from the machine dispenser of a typical European washing machine.
It has been attempted to spray a solution of the dye onto spray-dried detergent granules, but it has been found that the granules thus treated do not dissolve quickly enough to avoid dye staining of the fabric. The current trend is towards higher density detergent granules which are even less suitable carriers for the dye.
The best route of photoactivator dye addition available to date comprises spraying a dye solution onto low density granules that are subsequently dry mixed with the base granules of the detergent composition. Although these low density granules dissolve more quickly than the base granules they do not dissolve quickly enough to prevent dye staining of fabrics under various laundering conditions.